The conditions which are responsible for modulation of in vitro transformation will be investigated further, with specific emphasis on the use of UV, X-rays, combinations of carcinogens of different classes, the combination of in vivo - in vitro systems, and the effect of repair mechanisms on toxicity and transformation. Studies on the mechanisms of early events of carcinogen-cell interaction will be continued, as well as investigation of different proximate carcinogens and metabolic inhibitors. Special emphasis will be given to the requirements necessary to obtain neoplastic transformation of human fetal cells. The testing for the tumorigenicity of cells exposed to different agents for different time intervals will continue.